Analytical Study of Tunable Bilayered-Graphene Dipole Antenna

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1 1 Analytical Study of Tunable Bilayered-Graphene Dipole Antenna James E. Burke RDAR-MEF-S, bldg st floor Sensor & Seekers Branch/MS&G Division/FPAT Directorate U.S. RDECOM-ARDEC, Picatinny Arsenal, NJ Background: In the last decade, carbon allotropes have attracted the attention of the scientific community, first with carbon nanotubes and, since its isolation in 24, with graphene, which has shown unique electronic and physical properties, such as unconventional integer quantum hall effect, high carrier mobility at room temperature, and potential for a wide range of applications, like nanoribbon FETs[1]. The excellent transport properties of single and multilayer graphene hold promise to build ultrafast transistors with excellent on state characteristics. However, the lack of significant band gap in such systems has been one of the major roadblocks to achieve low off state current and hence high on/off current ratio. Recently, it has been found, both theoretically and experimentally, that a band gap can be opened up in a bilayer graphene (BLG) using an external bias [3]. Recently, theoretical models and experiments have shown that bilayer graphene has the interesting property of an energy gap tunable with an applied vertical electric field [1]. While the industry is progressing on making graphene a robust semiconductor to replace the other organic and inorganic semiconductors, graphene can also be implemented as a semiconductor antenna. Such applications would not necessarily be limited to the utilization of graphene in the ground plane of antenna structures as has been the focus of much of the current research, but rather focusing on utilization of graphene transmission line structures for antenna applications. Such antenna configurations can be integrated readily with other semiconductor devices in various applications including high-resolution airborne radar [2]. Objectives: The focus of this research is the mathematical study of the characteristics of a planar microstrip dipole antenna with a bilayer graphene semiconductor as the transmission line. In any microstrip structure, there is a transmission layer, dielectric layer, and ground layer. In this study, there will be a high impedance metal layer on top of the transmission line in segments along the length of the antenna. Like a double gate contact of a MOSFET transistor, this high impedance layer and ground plane will provide a vertical electric field to create a bandgap in the BLG layer. This study will use bandgap tuning in the BLG to provide theoretical data on tuning a dipole antenna in different sequences along the antenna length. Selected sequences will be chosen in this study to determine the change in radiation patterns and magnitudes in the far field region The results of the data in this paper can benefit radar, munition proximity fuzes, and mobile wireless communication devices. This study will use experimental data from [4] to provide bandgap change due to electric displacement. Design and Structure: The design of the antenna involves a two atom thick sheet of carbon known as bilayer graphene. The graphene is used as a transmission line on a half wavelength 15 GHz dipole microstrip antenna. The length of the antenna and wavelength were chosen considering the size of BLG material available in today s industry. The BLG transmission layer is separated from the high impedance contact top layer and the ground plane by a1 nanometer thick dielectric material with a dielectric constant of 1.8, as shown in Figure 1 (right). The width of

2 Report Documentation Page Form Approved OMB No Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 124, Arlington VA Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 3 MAR REPORT TYPE Technical Report 3. DATES COVERED to TITLE AND SUBTITLE Analytical Study of Tunable Bilayered-Graphene Dipole Antenna 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) James Burke 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) U.S. Army RDECOM-ARDEC,Sensors & Seekers Branch,Bldg.94 south, 1st floor,picatinny Arsenal,NJ, SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) U.S. Army RDECOM-ARDEC, Sensors & Seekers Branch, Bldg.94 south, 1st floor, Picatinny Arsenal, NJ, PERFORMING ORGANIZATION REPORT NUMBER 1. SPONSOR/MONITOR S ACRONYM(S) 11. SPONSOR/MONITOR S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT The focus of this research is the mathematical study of the characteristics of a planar microstrip dipole antenna with a bilayer graphene semiconductor as the transmission line. In any microstrip structure, there is a transmission layer, dielectric layer, and ground layer. In this study, there will be a high impedance metal layer on top of the transmission line in segments along the length of the antenna. Like a double gate contact of a MOSFET transistor, this high impedance layer and ground plane will provide a vertical electric field to create a bandgap in the BLG layer. This study will use bandgap tuning in the BLG to provide theoretical data on tuning a dipole antenna in different sequences along the antenna length. Selected sequences will be chosen in this study to determine the change in radiation patterns and magnitudes in the far field region The results of the data in this paper can benefit radar, munition proximity fuzes, and mobile wireless communication devices. 15. SUBJECT TERMS bilayer graphene, tunable graphene, tunable antenna, dipole antenna, microstrip antenna, semiconductor antenna 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 3 a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified 18. NUMBER OF PAGES 9 19a. NAME OF RESPONSIBLE PERSON Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18

3 2 the antenna is 1, micron. The high impedance contact layer on top of the BLG is divided along the length of the antenna into 31. micron segments separated by 2.5 micron wide gaps between each contact in the x-axis as shown in Figure 1 below. The ground plane of antenna provides the coupling and grounding of the high frequency signal, as well as the grounding of the DC bias voltage. The material of the ground plane is a low impedance metal for reflection. The gaps will be considered negligible in this study for simplifying calculations, and because the gap is 1/8, th of a wavelength. Each x segment will be calculated in the next section of this study as microns for the unit length. The dipole gap at the center of the transmission line is microns wide. Figure 1 displays a portion of the 32 total impedance contacts along the length of the antenna. The quantity of 32 was chosen to easily control the tuning of the antenna and contribute to future applicable tuning like: 32 Bit digital tuning and PCM (Pulse Code Modulation). Since the quantity of the high impedance gate contacts determines the unit length calculations, the disadvantage will mean fewer data points. Figure 1, (left) isometric view of microstrip BLG dipole antenna with center bottom feed of high frequency signal and DC biasing of top contacts and ground, (right) front view section cut of the bilayered graphene microstrip dipole antenna Calculations: The experimental data provided in reference 4, has bandgap changes in a BLG transistor from ev to.25ev as the vertical electric displacement in double gate contacts increases from to 3. V/nm. The experimental data in reference 4 uses the same bilayered graphene and dielectric material thicknesses as this study. With the known values of the electric displacement, dielectric constant, and dielectric thickness, the density of states or DOS within the bilayer graphene can be determined. DOS was calculated using the equation from reference 1 below: (1) With the being the conduction band energy from Fermi level, being reduced Planck s constant, being the effective mass, and being the minimum Dirac point of the BLG. The and are determined in reference 1 using an interlayer hopping energy of -.365eV within the BLG and an in-plane hopping energy of -3.33eV. The DOS decreases from 6.82 x 1 19 cm -2 to 3.1 x 1 19 cm -2 as the electric displacement changes from to 3.V/nm. Translating the electric displacement to gate voltages provides a range of.84v to 1.V. The gate

4 3 voltages are under a flatband assumption within the BLG layer. The electron concentration of the BLG as the gate potential between the high impedance layer and ground plane increase are calculated in the equations below. The electron mobility for BLG is assumed to be 1, cm 2 /Vs. The T represents the temperature in Kelvin, which for this study will be at room temperature. The k B, represents the Boltzmann s constant. Knowing the permittivity of the dielectric layers, susceptibility of BLG, and antenna width, the total impedance per unit length and current density can be determined using the equations below. The unit length x is microns as mentioned in the previous section of this study. The second and third terms in equation (3) represent the inductance impedance and capacitance impedance, respectively. Equations (3) and (4) calculate the change in impedance across the antenna length as the DC bias changes the conductivity per unit length. The results of the impedance on one unit length as the conductivity changes are in Figure 4. The total impedance across the antenna is calculated in unit lengths across a quarter of a wavelength in each x-direction of the center feed as described in Figure 1 above. (2) (3) (4) Where W, represents the width of the BLG antenna and V, represents the initial input RF signal amplitude at a value of 1V. The f is the frequency of the signal, d is the 1nm thickness of the dielectric. The t BLG represents the.8nm thickness of the BLG transmission layer. Equation is RF signal at the center feed directly proportional to the impedance across the antenna length to yield the current density. Equation (6) determines the far field intensity at different elevation angles as different DC biases are applied to all 32 contacts simultaneously. With equation (7), the far field intensity per unit length can be calculated for different sequences of DC biases applied along the antenna. The k represents the wave number of the signal and Z is the air impedance. (5) (6)

5 4 The dl in equation (7) above is equivalent to (x) length in this study. The different sequences of applying vertical fields to each of the 32 DC contact segments are decrease the DC bias on each contact from the center of the dipole to the each end, and increasing the DC bias on each contact from center dipole to each end. Results: The tool used in this study is MATLAB R29A conducting all the calculations of the above equations. As mentioned in the design and structure section of this study, experimental data from reference 4 contains 7 data points of the bandgap of bilayered graphene increases as the electric displacement increases. When implementing that experimental data into the equation (1) and (2), the electron concentration in Figure 2 decreases as the DC bias increases. This verifies typical semiconductor behavior with the bandgap increasing reducing the ability of electrons to jump into the conduction band. The electron concentration being inversely proportional to the electron mobility and electron charge can yield the conductivity of the bilayered graphene material shown in Figure 3 below. (7) 4.5 x 119 Change in Concentration Due to Increase in Bandgap Electron Concentration, N, (electrons/cm 2 ) Bandgap DC Bias, (Volts) Figure 2, 2DEG electron concentration of the bilayer graphene material as the gate voltage is changing the bandgap of the bilayer graphene. As the bandgap increases the concentration drops.

6 5 2.5 x 17 Conductivity of BLG as Bandgap Increases 2 Conductivity, (S/cm) Bandgap DC Bias, (Volts) Figure 3, conductivity change as the bandgap increases in the bilayered graphene transmission line The conductivity of the BLG material along with the other parameters of the microstrip dipole antenna are fed into equations (3) and (4) for calculating the impedance per unit length across the entire half wavelength antenna. Figure 4 shows the change in impedance across unit length as the DC bias increases. 12 Impedance of BLG Segment as the Bandgap Increases 1 Impedance of BLG Segment, Z, (ohm) Bandgap DC Bias, (Volts) Figure 4, impedance per unit length at the gate voltage increases

7 6 The impedance data displayed in Figure 4 includes the inductance and capacitance over one unit length as the DC voltage increases. The maximum electric displacement in the experimental data is 3.V/nm, which produces.25ev gap from Fermi level to conduction band, translates to 1.V producing 1kohm/per unit length as shown in Figure 4 above. Feeding equation (3) and (4) per unit length across half the antenna length at different DC bias amplitudes into (5) yield the current density results in Figure 5 below. Each line in Figure 5 represents the different DC amplitudes that change the bandgap of the BLG material. 35 Current Density Along Half of the Antenna Length 3 25 Vt = V Vt =.84V Vt =.33V Vt =.48V Vt =.63V Vt = 1.V Current Density, I(x), (A/cm) Half Antenna Length, (cm) Figure 5, Current Density along the half of the dipole antenna The results of (6) are displayed in Figure 6 and 7 below. Figures 6 and 7 show the maximum normalized magnitude and the minimum normalized magnitude, respectively. The radiation intensity decreases in magnitude, but stays constant in beam width as the DC bias increases on all top contacts simultaneously. Data is show from elevation angles of to 18 degrees due to microstrip structure with a low impedance ground plane at the very bottom. The calculated results of (7) are displayed in Figure 8 and 9 on the different sequences will be displayed in non-polar plot give that the far field intensity changes per unit length. The results have shown so far that the impedance increases and the DC bias increases, and the impedance decreases as the DC bias decreases. Figure 8, is the DC bias decreasing from the center feed to the each end of the antenna on each of the 32 top contacts. Figure 9, shows the results of the DC bias increasing from the center feed to the each end of the antenna on each of the 32 top contacts. At 3 degrees elevation the magnitude of the intensity is minimum and at 9 degrees elevation the intensity is maximum. Both plots in Figure 8 and 9 are normalized by the maximum intensity with no DC bias. The results show that the DC bias decreases from center to each end the beam width gets wider, and the beam width gets narrower when the DC bias increase from center to each end.

8 Figure 6, far field intensity vs. elevation angle radiation pattern normalized by maximum intensity with no DC voltage applied to BLG e e-5 4e-5 3 2e Figure 7, far field intensity vs. elevation angle radiation pattern with a DC Voltage of 1V applied to all the top contacts. This plot is normalized by maximum intensity with no DC voltage.

9 8.35 Radiation Intensity as Bandgap Decreases From Center to End.3 3deg Elevation 6deg Elevation 9deg Elevation Far Field Intensity, du,(w/cm 2 ) Antenna Length, L, (cm) Figure 8, far field intensity vs. length of BLG microstrip dipole antenna with the gate voltages decreasing from center to end.14 Radiation Intensity as Bandgap Increases from Center to End.12 3deg Elevation 6deg Elevation 9deg Elevation.1 Intensity, du, (W/cm 2 ) Antenna Length, L, (cm) Figure 9, far field intensity vs. length of BLG transmission line with DC voltage increases from center to end

10 9 Conclusions: The results of this study show that it is possible to tune a bilayered graphene as an antenna in a microstrip structure. The results show that applying vertical DC fields to the outer top contacts can narrow the beam width of the dipole radiation, and widen the beam by applied DC fields to the inner top contacts. Since the magnitude of the radiation decreases by a factor of 1 6 from V DC to 1V DC, a binary signal can be used to tune the antenna by just applying 1 and bits. Since there are 32 total top layer gate contacts, a 32 bits digital signal could be used to tune this type antenna. Future studies will determine how to apply a digital signal along with high microwave input signal to tune the antenna for filtering, beam steering, or beam forming. The benefits of this research can be an asset to smart munitions in the military and mobile wireless communications devices in the commercial industries. Reference: 1 Cheli, Martina, A Semianalytical Model of Bilayer-Graphene Field-Effect Transistor IEEE Transactions on Electron Devices, VOL. 56, NO. 12, December 29 pp Dragoman, M. Terahertz Antenna Based on Graphene Journal of Applied Physics 17, (21) pp Jain, F.C., Semiconductor Antenna: A New Device in Millimeter- and Submillimeter-Wave Integrated Circuits IEEE Transactions On Microwave Theory and Techniques, VOL. MTT-32, NO. 2, February 1984 pp Majumdar, Kausik, Intrinsic limits of subthreshold slope in biased bilayer graphene transistor Applied Physics Letters 96, (21) pp R. Simon, Antennas and Propagation for Wireless Communication Systems, Second Edition 27 John Wiley & Sons, Ltd pp